Bone cancer pain significantly decreases the quality of life and functional status for millions of cancer patients each year. Currently, our vision of how sensory nerve fibers change when tumors metastasize and grow in bone is that nerve fibers are first sensitized and activated by factors released by tumor/stromal cells, then injured as tumor and stromal cells proliferate and remodel the tumor bearing bone. However, preliminary data we have generated suggests that tumor and tumor-associated stromal cells also induce dramatic sprouting and neuroma formation of sensory and sympathetic nerve fibers that innervate the bone. Our hypothesis is that tumor and stromal cells induce a marked reorganization of TrkA+ nerve fibers and that the pathological reorganization of these nerve fibers plays a significant role in driving bone cancer pain. Based on these observations, we hypothesize that: (1) nerve growth factor (NGF) released from tumor and stromal cells induces marked sprouting and neuroma formation in TrkA+, but not TrkA-, sensory and sympathetic nerve fibers; (2) newly sprouted sensory nerve fibers have a distinct morphology and pathologically high expression levels of neurotransmitters, ion channels, receptors and mitogen-activated protein kinases, which is different from nerve fibers that innervate the normal bone, and (3) early administration of anti-NGF or TrkA antagonist will block these pathological changes and the severity of bone cancer pain more effectively than late administration. The overarching hypothesis is that the earlier administration of anti-NGF or TrkA blockade is begun, the more likely these therapies will block tumor-induced nerve sprouting, neuroma formation, inappropriate up-regulation of ion channels, and pain. If correct, data generated from this project has the potential to fundamentally change our understanding of the mechanisms that drive bone cancer pain and promote the use of preventive analgesia for managing bone cancer pain.